The present invention is directed to quartz (alternatively referred to as silica) crucibles and more specifically to a method for forming fused quartz crucibles used in the semiconductor industry for growing single crystal silicon articles.
In the production of silicon crystals, the Czochralski method is often employed where polycrystalline silicon is first melted down within a quartz crucible. After the polycrystalline silicon has melted, a seed crystal is dipped into the melt and subsequently extracted while the crucible is rotated to form a single crystal silicon ingot.
It is important that the crucibles used for preparing single crystal silicon, particularly for the semiconductor industry, be essentially free of impurities. In addition, the quartz crucible is preferably largely free of included bubbles and other structural imperfections.
Accordingly, it is highly desirable to have available a method for forming silica crucibles having a low bubble content and/or bubbles containing gasses having little or no negative effect on silicon crystal growth.
Traditionally, to prepare such crucibles, a raw material quartz is introduced into a rotating hollow mold which has gas pervious wall regions at the side and bottom. After introducing the raw quartz material, a heat source is introduced into the mold to melt the quartz. During heating, a vacuum is applied to the outside of the rotating mold to draw out interstitial gases. Although this process reduces bubble content, bubbles nonetheless remain. Moreover, while attempts have been made to control the atmosphere, a satisfactory process does not exist to adequately meet the unique requirements of quartz crucible manufacture.
Therefore, a need still exists in the semiconductor industry for a crucible with a low bubble content and/or a crucible having bubbles containing gases with little destructive effect on the crystalline melt and the drawn crystal. This can be accomplished by reducing the number of bubbles and by controlling the composition of the gas trapped in the bubbles to reduce impact on crystal growth. Moreover, the composition of the gas in the bubbles can be controlled such that the included gas is highly soluble in the silicon melt and evolves therefrom. One important requirement to achieve this goal is the availability of a method for crucible manufacture which facilitates production of crucibles with reduced bubble count and with controlled bubble gas composition.
According to an exemplary embodiment of this invention, a method for the manufacture of quartz crucibles is provided. The method comprises providing an at least substantially enclosed chamber surrounding a crucible mold. The atmosphere in the chamber is controlled at a positive pressure wherein the total outflow of gas is less than the total inflow of gas. Once the desired atmosphere has been achieved, a heat source is provided (usually an arc) to melt a quartz material lining the inside of the mold, allowing the quartz material to fuse.
According to an alternative embodiment, a method for quartz crucible formation comprising controlling the atmosphere surrounding a crucible mold is provided. The atmosphere is controlled by replacing an ambient atmosphere with an atmosphere comprising desired gases. The outflow of the desired gases is monitored at a point after the desired gases have passed through the crucible mold. The crucible is fused after the desired gases have been identified in the monitoring step.